U.S. Pat. Nos. 4,910,072, 4,900,618 and 5,082,734 disclose selective catalytic activation of catalytically inert polymeric films having essentially no metal on the surface. Such selectively activatable films are prepared from solutions of polymer and noble metal compounds. Maintaining adhesion of the catalyst to the surface is of considerable importance since loosely adhered catalytic metal can be washed from the surface in the agitation of the plating bath causing depletion of the metal value of the plating bath as uncontrolled metal deposition occurs, resulting in what is commonly referred to as a "crashed" bath. Adhesion of the catalytic layer to the substrate also affects the ultimate adhesion of the plated metal. Compatibility of a catalytic polymer film with a substrate, e.g. in terms of surface tension, hydrogen bonding or mechanical adhesion, etc., can be important in providing a strongly adherent metal layer. Certain polymer substrates, e.g. parts having sufficient thickness, are commonly etched or swelled at the surface to provide anchoring sites for colloidal palladium catalyst. Due to their chemical and thermal resistant polyimide film are perceived to be desirable substrates for flexible circuitry applied by electroless deposition. Such chemical and thermal resistance provide unique challenges to appropriate surface treatment that will allow adherent metal coatings. Among the inventive solutions to this technical challenge is flame treatment as disclosed in U.S. Pat. No. 3,954,570. Alternatively, polyimide films have been treated with caustic solutions to generate amic acid salt groups on the surface followed by application of solubilized or colloidal palladium as disclosed in U.S. Pat. Nos. 3,937,857, 4,992,144, 5,178,914 and 5,183,692. A mechanical means of providing a porous surface is disclosed in U.S. Pat. No. 4,847,139 where a copper laminated coating is removed from a polyimide film by etching to provide surface amenable to conventional plating pretreatment.
One difficulty encountered in the use of films of polyamic acid and noble metal catalysts results from the tendency of palladium to catalyze the oxidation of polyamic acid at elevated temperatures typically required to effect imidization, e.g. at about 250.degree. C. and higher in the presence of oxygen.